Why is forecast performance measured over a designated period rather than in mon

Why is forecast performance measured over a designated period rather than in monthly periods?
Bias: What does a negative bias mean in terms of actual demand and what does a positive bias mean in terms of actual demand? What does a bias of zero represent? Explain the effects on distribution and warehousing when a bias indicates the forecast is consistently over or under.
Tracking signals: What does it mean to a forecaster when the tracking signal is tripped (considered both a positive and negative trip value? Explain the impact in a distribution or warehouse system if a tracking signal is tripped and the forecast plan is not adjusted.
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